This disclosure relates to a method of manufacturing garnet interfaces and to articles containing the garnets obtained therefrom.
A scintillation detector or scintillation counter is obtained when a scintillator material is coupled to an electronic light sensor such as a photomultiplier tube (PMT), photodiode, silicon photomultiplier, and the like. Photomultiplier tubes absorb the light emitted by the scintillator material and convert it to an electron current via the photoelectric effect. The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and which yields meaningful information about the particle that originally struck the scintillator material. A scintillator is a material that produces light when excited by ionizing radiation. Luminescent materials when struck by an incoming particle, absorb its energy and scintillate, (i.e., re-emit the absorbed energy in the form of light).
A useful characteristic of a scintillator material is the amount of light produced in a scintillation process, which can be measured as a number of scintillation photons produced by the absorption of 1 MeV energy of an ionizing particle. Only a small fraction of the scintillation photons produced in a scintillation event reach the detector. A substantial number of the scintillation photons are lost by absorption or by losses at optical interfaces in the detector module.
One of the ways to improve the efficiency of light collection is to improve the optical clarity of the scintillator material by improving crystal uniformity. This may result in the reduced absorption of scintillation photons and photon transfer properties of the material. Another method is to change the angular exit distribution (the distribution of exit angles) at which the photons exit the surface of the scintillator material.
In timing applications, it is desirable to minimize the number of reflections that occur at the scintillator material exit surface and thus reduce the dispersion of photon arrival times at the light sensor. This may be achieved by modifying the exit surface of the scintillator material by forming a micro-structure of well-defined grooves. By using these groves it is possible to extend range of acceptance angles of incident photons propagating at the exit of the scintillator. FIG. 1 depicts one manner of disposing grooves on the exit surface of the scintillator to reduce the number of photons that are reflected back from the interface due to total internal reflection. FIG. 1 shows a block of glue 106 disposed on an exit surface of a lutetium orthosilicate scintillator (LSO) crystal 102. Also disposed on the exit surface of the scintillator material 102 are micro-textures (i.e., grooves) in the form of an array of pyramids 104. The presence of the pyramids reduces the amount of total internal reflection that would have occurred if the exit surface of scintillator material 102 was flat.
The effect caused by the presence of the pyramids on the exit surface of the scintillator material can be demonstrated by using ray tracking simulations of the scintillator 102 with and without the pyramids 104 as shown in the FIG. 2(A) and the FIG. 2(B) respectively. The FIG. 2(A) depicts the scintillator 102 as having a flat exit surface. Because of this flat exit surface some of the photons that impinge on the surface at an angle that is greater than or equal to the critical angle are reflected completely at the interface back into the scintillator material 102.
FIG. 2(B) depicts a scintillator material 102 surface that is textured with pyramids 104. The pyramids 104 permit a higher percentage of the photons, which have a distribution of incident angles affected by the shape of the scintillator 102 and any surrounding optical elements, to pass through the interface because they are incident upon the interface at angles that are less than the critical angle.
A variety of methods can be used to manufacture the textured exit surface of the scintillator material, such as, for example, mechanical polishing, laser cutting, chemical etching, or even bonding additional structures with a refractive index closely matching that of the scintillator. The latter method is particularly difficult due to the fact that most scintillator materials used for high energy applications have relatively high refractive indices when compared with most optical adhesives used as optical coupling agents. In this case, light reflection losses occur at the optical interface where a significant number of scintillation photons are lost. Moreover, creating such a structure on the surfaces of scintillator materials by methods such as mechanical polishing, laser cutting, or chemical etching produces another problem. High stresses generated during the fabrication of the scintillator material surface result in cracking or crazing of the crystal surface. It is therefore desirable to develop methods for producing scintillator materials with textured surfaces (that can be used to preserve the incident photons) and that do not degrade with time.